Measuring based on a capacitive acceleration sensor has proved to be an acceleration measuring method of a simple and reliable principle. Capacitive measuring is based on a change in the gap between the two surfaces of a pair of electrodes of the sensor. The capacitance between the surfaces, i.e. the capacity for storing electric charge, depends on the area of the surfaces and the distance between them. Capacitive measuring can be used even at quite low metering ranges of acceleration values.
Generally, small capacitive acceleration sensor structures are based on micromechanical structures manufactured on silicon. The micromechanical structures are typically structures having a thickness exceeding 100 μm, formed by etching a wafer material. An advantage of micromechanical capacitive acceleration sensors is the large mass in relation to the area of the structures, which enables the manufacturing of capacitive acceleration sensors of excellent performance.
Connection and encapsulation methods of prior art presently used in the manufacturing of professional and consumer electronics and the miniaturization of consumer electronics have led to tight requirements regarding the size of micromechanical components, such as capacitive acceleration sensors, and, in particular, regarding the height of the sensor components.
Presently, some prior art capacitive acceleration sensor solutions measuring in relation to several axes are known. Such solutions are described in, for example, the German Patent Announcement Publication DE 10225714, and in U.S. Pat. No. 6,829,937. The acceleration measuring principles described in the publications are based on an asymmetrical support of a mass by means of a torsion spring, such that perpendicular to the spring axis passing through the center of gravity of the mass forms an angle of essentially 45° with the capacitor plates.
Below, prior art will be described with exemplifying reference to the accompanying drawings, of which:
FIG. 1 shows a capacitive acceleration sensor solution, according to prior art, in section and projection view,
FIG. 2 shows a positioning solution of capacitive acceleration sensor elements, according to prior art, for measuring acceleration in relation to three axes,
FIG. 3 shows a second capacitive acceleration sensor solution, according to prior art, in section and projection view, and
FIG. 4 shows the influence on the distance between the measuring electrode and the mass, from deformation in the second capacitive acceleration sensor solution, according to prior art.
FIG. 1 shows a capacitive acceleration sensor solution, according to prior art, in section and projection view. In the capacitive acceleration sensor solution, according to prior art, the torsion springs 4 supporting the mass 1 of the moving electrode are positioned off-center in the longitudinal direction and at one edge of the mass 1 in the thickness direction. The measuring electrodes 2, 3 are positioned below the mass, symmetrically in relation to the spring axis. In the projection view, the areas of the mass 1, which coincide with the measuring electrodes 2, 3, are depicted by dotted lines.
FIG. 2 shows a positioning solution of capacitive acceleration sensor elements, according to prior art, for measuring acceleration in relation to three axes. In the positioning solution according to prior art, the acceleration sensor elements 7, 8, 9 are positioned for measuring acceleration in relation to three axes. By means of the depicted positioning solution, an acceleration sensor of several axes can be implemented, the directions of measuring of which tune the entire space.
An advantage of the capacitive acceleration sensor solution, according to prior art, shown in FIGS. 1 and 2, is the position of the electrodes in the same plane, and the immunity to deformations of the structure. Deformations of the structure are almost unavoidable, when the sensor is being mechanically and electrically connected, and when it is being protected against chemical influences from the environment. These deformations are caused by differences in the thermal expansion of the materials. The capacitive acceleration sensor solution according to prior art, described above, withstands deformations of the structure extremely well without generating measuring inaccuracy due to null shift.
An advantage of the capacitive acceleration sensor solution, according to prior art, shown in FIGS. 1 and 2, is also, that adjusting the vertical and horizontal sensitivities of the acceleration sensor solution is easily done by changing the angle of the perpendicular to the spring line passing through the center of gravity. If the angle is larger than 45°, a vertical sensitivity smaller than the horizontal sensitivity is achieved, which is useful in many practical applications.
A disadvantage of the capacitive acceleration sensor solution, according to prior art, shown in FIGS. 1 and 2, is the ineffective use of space, as some part of the surface of the mass remains uncovered by electrodes.
FIG. 3 shows a second capacitive acceleration sensor solution, according to prior art, in section and projection view. In the second capacitive acceleration sensor solution, according to prior art, the torsion springs 13, which support the mass 10 of the moving electrode, are positioned in the corners of the mass 10. The measuring electrodes 11, 12 are positioned in two different planes on both sides of the mass 10. In the projection view, the area of the mass 10 coinciding with the measuring electrodes 11, 12 is depicted by a dotted line.
Also in the solution of FIG. 3, the measuring direction is angled by 45° off the plane of the measuring electrodes 11, 12. Asymmetry has been achieved by positioning the torsion springs 13 in the corners of the mass 10. An advantage of the second capacitive acceleration sensor solution, according to prior art, shown in FIG. 3, is an extremely efficient use of the area.
A disadvantage of the second capacitive acceleration sensor solution, according to prior art, shown in FIG. 3, is the position of the measuring electrodes 11, 12 in two planes located far away from each other. The electrodes 11, 12 located in two different planes require great rigidity in the entire structure.
FIG. 4 shows the influence on the distance between the measuring electrode and the mass, from deformation of the second capacitive acceleration sensor solution according to prior art. In the second capacitive acceleration sensor solution, according to prior art, the torsion springs 18 supporting the mass 15 of the moving electrode are positioned in the corners of the mass 15. The measuring electrodes 16, 17 are positioned in two different planes on both sides of the mass 15.
A disadvantage of the second capacitive acceleration sensor solution, according to prior art, shown in FIG. 4, is the disproportion in the distance between the measuring electrodes 16, 17 and the mass 15 caused by bending or some other deformation of the acceleration sensor structure. Deformations of the structure are almost unavoidable, when the sensor is being connected mechanically and electrically and when it is being protected against chemical influences from the environment. These deformations are caused by differences in the thermal expansion of the materials.
FIG. 4 shows how the distances between the mass 15 and the measuring electrodes 16, 17 located on different sides of the mass 15 of the capacitive acceleration sensor solution, according to prior art, change in a mutually different manner, as the sensor bends. The consequence of this is, that the difference between the two capacitances of the sensor changes, i.e. a measuring error caused by a null shift is generated. The situation for the second capacitive acceleration sensor solution, according to prior art, is further complicated by the fact, that the measuring electrodes 16, 17, located on different sides of the mass, can bend independently of each other, if the loading forces and moments are asymmetrical.
A problem with the acceleration sensor solutions according to prior art is the excessive height of the finished sensor component. Nowadays, acceleration sensors are required to have a small area and a good performance.
An advantage of a low height is good installability in modern electronic products. Correspondingly, an advantage of a small area is a low production cost in the wafer process. Further, a good performance often means low noise during the measuring and stability in the device during the measuring. The performance often requires rigidity of the structures.
The capacitive acceleration sensor structure, according to prior art, shown in FIG. 1, wastes area regarding the area of the capacitors. Instead, the presented solution tolerates mechanical deformation, and thus, it can be designed to have much thinner support structures than the solution of FIG. 1.
By means of the second capacitive acceleration sensor solution, according to prior art, shown in FIG. 3, an optimal compromise is achieved regarding the small area required by the manufacturing costs and the large capacitor area required by the performance. The presented solution is, however, particularly bad, when looking for a compromise between thickness and structure rigidity. The capacitor plates located in different planes require very great rigidity of the entire structure.
In the manufacturing of professional and consumer electronics, there is a clearly growing requirement for lower capacitive acceleration sensors than earlier solutions, sensors applicable for use for reliable acceleration measuring particularly in small capacitive acceleration sensor solutions measuring acceleration in relation to several axes.